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it's unlikely to make any difference

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... except to your bank balance. The going rate for a decent filter on the high street seems to be around

In reply to:

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The going rate for a decent filter on the high street seems to be around

PlusNet MaxDSL
I used to have a handle on life, but it broke.

Just to give the corrects facts one more vote.

An ADSL modem/router does NOT need a filter. Admittedly a filter is handy as it will ensure the correct connection, but if you open a filter up and trace out the pcb, you will see that the modem RJ-11 connection - which is the middle two wires/contacts is connected to contacts 2 & 5 of a 6-way BT connector.
It is for this reason that simply crimping a BT plug on one end, and a RJ-11 on the other of a piece of flat phone cable will NOT give the correct connections. Whereas a proprietory cable with BT on one end and RJ-11 on the other,sold for conecting a modem, any type - Analoge or ADSL - will work.

Don't waste money on "posh" ones, either.